what color is the car originally, and what color are you trying to make it?
when i first started, until i figured things out i couldn't do anything to a car that was grey, white, or black. darn shades.

use the pen tool, or the polygon lasso to select all the painted parts of the car, click image, adjustments, desaturate, (shift,control,U) this will turn the car colorless. Then click image, adjustments, variations, this will bring up a tool window. Click the color you want to make the car, clicking more than once on the color will saturate it with more color, clicking more than one color will mix custom blends. Hope that helps.